Conventional s-Wave Superconductivity in Noncentrosymmetric Ir2Ga9: 71Ga-NQR Evidence

Superconductivity without spatial inversion symmetry has begun to attract much attention since the discovery of the superconductor CePt3Si. 1) It is presumed that the absence of spatial inversion symmetry causes an admixture between even and odd parities of superconducting pairing due to antisymmetric spin–orbit coupling (ASOC). However, there has been no clear evidence of the admixture thus far, and hence the spatial noncentrosymmetric effect on superconductivity remains experimentally controversial. Recently, ASOC has also been discussed in transition-metal superconductors. In this system, we can consider ASOC without strongly correlated electron systems, which would be appropriate for tracing the effect of ASOC. Interestingly, in Li2Pt3B, it was reported that unconventional superconductivity with a line node and a spin-triplet pairing state emerge, which are ascribed to ASOC. These reports suggest that ASOC can induce a novel superconducting state even though there is no strong electron correlation. The intermetallic binary Ir2Ga9 does not possess spatial inversion symmetry and becomes superconducting below Tc 1⁄4 2:2K. Specific heat and resistivity measurements showed that the compound is a weak-coupling BCS superconductor with an isotropic gap, and located near the boundary between type-I and type-II superconductivities with an upper critical field Hc2 150Oe. In this letter, we report on the characteristics of superconductivity in noncentrosymmetric Ir2Ga9 probed by Ga-nuclear-quadrupole-resonance (NQR) measurement at a zero field (H 1⁄4 0). The spatial noncentrosymmetric effect on the superconducting state is discussed. A single crystal of Ir2Ga9 was grown by the Ga flux method. Powder X-ray diffraction indicated that the compound forms in the primitive monoclinic Rh2Ga9 type structure. The sample of Ir2Ga9 was crushed into coarse powder for NQR measurement to allow the penetration of the rf field. The NQR measurement was performed by the conventional spin-echo method in the temperature (T) range of 1–280K. T1 was measured at f 26:45MHz, which was the 1 Qð 1=2 $ 3=2Þ transition of Ga (I 1⁄4 3=2), as shown in Fig. 1. The NQR spectrum ensures the quality of the sample, because the full width at half maximum (FWHM) is as small as 54 kHz, as shown in Fig. 1. The NQR signal of the isotope Ga (I 1⁄4 3=2) was confirmed at f 42:08MHz, which was consistent with the ratio 69Q=Q 1:59 (Q is the quadrupole moment) because NQR frequency is proportional to Q. Figure 2 shows 1=T1T as a function of T in the temperature range of 1 T 280K along with the ac-susceptibility measured using an in-situ NQR coil. In the normal state above Tc, a T1T 1⁄4 const. relation is valid up to T 50K, which is usually seen in conventional metals. Above T 50K, although 1=T1T gradually deviates from the constant, it is probably ascribed to the local structural distortion of the crystal because NQR frequency also shifts in the same T range, as seen in the inset of Fig. 2. This structural change may bring about an increase in the density of states at the Fermi level, leading to an increase of the 1=T1T constant value in the high T region. It is important to investigate the precise crystal structure of Ir2Ga9 at a low T where the superconductivity emerges. In the superconducting state, NQR intensity is largely suppressed below Tc 1⁄4 2:2K, as shown in the inset of Fig. 1. This is consistent with the previous reports that 26.2 26.3 26.4 26.5 26.6 26.7 26.8 N Q R in te ns ity ( ar b. u ni t)